and 180 feet below, 21st century citizens look up at washington rising at the center of the city laid down by he and urban planner pierre l'enfant. >> ayers: look how l'enfant laid out this city with these radiating streets-- maryland and pennsylvania and new jersey and delaware. >> pelley: the capitol right in the center... >> ayers: right in the middle. >> pelley: ...and everything proceeds from here. >> ayers: and you put the capitol atop this hill. and that's how you create this iconic symbolism. >> pelley: it was a symbol for the whole nation that the union would endure. now, your job is to make the dome endure. up here, you can see what is hidden from a distance. the dome is falling apart. your guys bring in pieces into the office and say, "hey, boss, here's what i found"? >> ayers: you can see the level of rust behind it that is just eating away at the cast iron. >> pelley: even worse, there are more than 1,300 serious cracks in the dome. architect stephan ayers is starting a massive repair project. he showed us how they'll cover the dome with scaffolding for the next two years